Physical properties of an adhesive may be evaluated in terms of tack, adhesion and cohesion. These properties are affected by the kind of monomers, molecular weight, molecular weight distribution, and additives. Among them, tack and adhesion increase as the molecular weight is smaller and as the glass transition temperature (Tg) is lower, i.e., as the adhesive is more flexible. In contrast, cohesion and holding power increase as the molecular weight is larger, i.e., as the secondary binding force is larger. Cohesion is an intermolecular attraction. A high cohesion results in an interfacial fracture of the adhesive during debonding, whereas a low cohesion will lead to a cohesive fracture, leaving fractured adhesive residue on the adherend. Thus, it is crucial to improve adhesion properties while causing an interfacial fracture.
Adhesive compositions including styrene-based block copolymers as main thermoplastic elastomer components are widely used as pressure sensitive adhesives for industrial tapes, packaging tapes and labels, and as hot melt adhesives for diapers, feminine hygiene products, or the like. Usually, poly(styrene)-poly(isoprene)-poly(styrene) (SIS) block copolymers and poly(styrene)-poly(butadiene)-poly(styrene) (SBS) block copolymers are commonly used as those adhesive composition.
Unlike the SBS block copolymers for other applications wherein the mechanical properties of them are required above all, processing properties such as viscosity and adhesion properties after manufacture are important for the SBS block copolymers used in adhesives. Thus, the first considerations when preparing adhesives using the SBS block copolymers have good compatibility with other materials such as adhesive resins, plasticizers (oils), etc., good heat resistance and low viscosity in view of processing. And, with regard to the adhesion properties, loop tack, 180° peel strength, holding power, or the like are also important. These properties are affected by the molecular structure and molecular weight of the SBS block copolymers.
U.S. Pat. No. 3,265,765 describes that a hot melt adhesive may be prepared using the linear SBS triblock copolymer. Although thus prepared triblock copolymer may have superior holding power, it does not have good processing properties or adhesion properties.
U.S. Pat. No. 4,163,764 discloses linear or radial SBS block copolymers prepared using various coupling agents for use as hot melt adhesive. Thus prepared SBS block copolymers have improved processing properties, but their adhesion properties are not considerably different from other similar linear SBS triblock copolymers. Therefore, for improvement of the processing properties and adhesion properties, adhesives including tapered block copolymers in part have been introduced.
U.S. Pat. No. 4,603,155 discloses a multiblock copolymer of a structure styrene-butadiene/styrene-butadiene/styrene-butadiene-styrene, with the butadiene block at the center portion having two or more taper blocks.
U.S. Pat. No. 4,874,821 discloses a styrene/butadiene-styrene-butadiene block copolymer, with the styrene/butadiene block being tapered.
U.S. Pat. No. 5,554,697 discloses a preparation method for a styrene1-butadiene/styrene2-styrene3 block copolymer by adding styrene to be included in the styrene2 and styrene3 blocks at once at a specific transition ratio of butadiene, and an adhesive composition relating thereto.
Of the above described copolymers, those having multiple taper blocks have superior processing properties and adhesion properties. However, their polymerization time increases considerably while their softening point of the adhesive drops considerably as the taper content increases. Further, the addition of styrene at a specific transition ratio of butadiene is different from the addition of a predetermined amount of styrene to be tapered and has difficulty in stably producing the block copolymer with controlled content and size of the tapered styrene blocks. Thus, the existing methods are restricted in satisfying both producibility and quality for commercial application.